Autotaxin (ATX), also known as ectonucleotide pyrophosphatase/phosphodiesterase family member 2 (ENPP2), is a secreted lysophospholipase D (lysoPLD) that cleaves choline from lysophosphatidylcholine (LPC) forming lysophosphatidic acid (LPA), a potent mitogen and motily factor that has been implicated in the pathophysiology of cancer (Liu et al., 2009) (Houben and Moolenaar, 2011) (Leblanc and Peyruchaud, 2014) and many other biological processes such as vascular development lymphocyte homing and inflammation (see, e.g., Van Meeteren et al., 2007) (Moolenaar et al., 2013) (Knowlden and Georas, 2014). LPA consists of a single fatty acyl chain, a glycerol backbone and a free phosphate group. The great variety of cellular and biological actions of LPA is explained by the fact that the six known LPA receptors show broad tissue expression and can couple to at least six distinct G proteins, which, in turn, feed into multiple effector systems (Choi et al., 2010).
ATX is processed along the classical export pathway and secreted as a catalytically active glycoprotein. ATX's major lipid substrate, LPC, is secreted by the liver and is abundantly present in plasma and interstitial fluids.
As previously indicated, ATX is implicated in cancer and numerous other disease states. The role of ATX in cancer and various other disease states is summarised below.
ATX and Cancer
ATX is widely expressed, with highest mRNA levels detected in lymph nodes, brain, kidney, testis, pancreas, lung and liver. ATX is found overexpressed in several common human cancers, while many established tumour cell lines express ATX to varying levels (see references above). Expression is also detected in stromal cells, including macrophages, fibroblasts and endothelial cells.
ATX is an attractive target for the treatment of cancer because it acts extracellularly and stimulates the metastatic cascade at multiple levels. In addition, ATX has been implicated in inflammatory processes by regulating lymphocyte homing (Kanda et al, 2008; Zhang et al, 2012; Knowlden and Georas, 2014).
ATX is thought to act in an autocrine/paracrine manner to promote tumour progression, i.e., by providing an invasive and angiogenic microenvironment for malignant cells. A causal link between the ATX-LPA axis and cancer is supported by a growing number of studies (for a review, see Van Meeteren et al., 2007; Houben A J, Moolenaar W H (2011). Cancer Metastasis Rev. 30:557-65.) (Leblanc and Peyruchaud, 2015).
Overexpressed ATX promotes tumour aggressiveness, metastasis and angiogenesis in mice (Liu et al., 2009).
ATX is overexpressed in various human cancers, including glioblastoma, lung and breast cancer, renal cell carcinoma and Hodgkin lymphoma. Furthermore, ATX is upregulated in stromal cells from cancer patients. (See, e.g., Zhao et al., 2007).
ATX mediates the EBV-induced growth and survival of Hodgkin lymphoma cells, while ATX knockdown reduces lymphoma cell growth and viability. (See, e.g., Baumforth et al., 2005).
Inducible overexpression of LPA1 receptors in breast carcinoma cells promotes tumour growth and bone metastasis, while LPA1 knockdown reduces tumour progression (Bouchabara et al., 2006).
ATX and LPA receptors have transforming potential both in vitro and in mice. (See, e.g., Taghavi et al., 2008. Liu et al. (2009) Cancer Cell. 15:539-50).
Inhibition of the LPA1 receptor reduces metastasis and metastatic dormancy in breast cancer. (Marshall et al., 2012).
Serum ATX levels in patients with B-cell neoplasms, especially follicular lymphoma (FL), are higher than those in healthy subjects (see, e.g., Masuda et al., 2008). Serum ATX in FL patients was associated with tumour burden and changed in parallel with the patients' clinical courses. Plasma LPA levels in FL patients correlated well with ATX levels. Since tumour cells from FL patients expressed ATX, secreted ATX from lymphoma cells probably underlies the increase in serum ATX. Thus, serum ATX is a promising marker for FL.
ATX/lysoPLD activity is also significantly elevated in malignant effusions from ovarian cancer patients. Furthermore, serum ATX activity decreases after prostate cancer surgery and may reflect postoperative damage or nutritional status. See, e.g., Nakamura et al., 2007.
Dual ATX and pan-LPA receptor inhibitors inhibit breast cancer cell migration and invasion and cause tumour regression in breast cancer xenograft model. (See, e.g., Zhang et al., 2009).
Overexpression of ATX or LPA receptors in breast cancer epithelium causes high frequency of late-onset mammary carcinomas. (See, e.g., Liu et al., 2009).
LPA2 knockout mice have reduced incidence of chemically induced colon carcinoma. (See, e.g., Lin et al., 2009).
ATX and Inflammation
High ATX expression is found in the high endothelial venules (HEVs) of lymphoid organs and in venules at sites of chronic inflammation, where it may play a role in T cell trafficking across the endothelial walls during inflammation. (See, e.g., Kanda et al., 2008). Intravenous injection of enzymatically inactive ATX attenuated the homing of T cells to lymphoid tissues, probably through competition with endogenous ATX. These results suggest that ATX is a potential target for anti-inflammatory therapy.
Along similar lines, Japanese investigators recently showed that injection of neutralizing monoclonal antibodies against ATX into mice reduced plasma LPA levels to zero. (See, e.g., Nakasaki et al., 2008). It thus appears that plasma LPA can be depleted by targeting ATX. These results suggest that ATX is a potential target for anti-inflammatory therapy.
ATX and Diabetes Melitus
ATX expression is significantly up-regulated in adipose tissue from patients exhibiting both insulin resistance and impaired glucose tolerance (see, for example, Boucher et al., 2005). This suggests that ATX may serve as a therapeutic target in obesity-associated type 2 diabetes (Nishimura S, et al. (2014). ENPP2 Contributes to Adipose Tissue Expansion and Insulin Resistance in Diet-Induced Obesity. Diabetes 63:4154-64).
ATX and Hypertension, Atherosclerosis and Thrombosis
LPA accumulates in the lipid core of human atherosclerotic plaques and is the primary platelet-activating lipid constituent of the plaques (see, for example, Siess et al., 1999). Furthermore, due to its ability to stimulate the proliferation of vascular smooth muscle cells, LPA may play an important role in the development of both hypertension and atherosclerosis (see, for example, Siess et al., 2004). Recent evidence shows that plasma ATX associates with platelets during aggregation and concentrates in arterial thrombus (see, for example, Pamuklar et al., 2009). Thus, unbalanced LPA homeostasis is a potential risk factor for thrombosis. Therefore, LPA-lowering ATX inhibitors may prove useful in the treatment of both hypertension and atherosclerosis.
ATX and Fibrosis
Mice lacking the LPA1 receptor are markedly protected from pulmonary fibrosis and mortality (see, e.g., Tager et al., 2008). The absence of LPA1 leads to reduced fibroblast recruitment and vascular leak, two responses that are excessive when injury leads to fibrosis rather than to repair. Thus, the ATX-LPA axis represents a therapeutic target for diseases in which aberrant responses to injury contribute to fibrosis, such as idiopathic pulmonary fibrosis, as well as renal interstitial fibrosis (see, e.g., Pradere et al., 2007), hepatic fibrosis and skin fibrosis.
ATX and Pain
Mice lacking the LPA1 receptor are also protected against injury-induced neuropathic pain and related behaviour (see, e.g., Inoue et al., 2004). Heterozygous Enpp2(+/−) mice, which have 50% ATX protein compared to wild-type mice, show approx. 50% recovery of nerve injury-induced neuropathic pain (see, e.g., Inoue et al., 2008). Therefore, targeting ATX (and its downstream LPA signaling pathways) represents a novel way to prevent nerve injury-induced neuropathic pain.
ATX and Urethral Obstructive Disease
Smooth muscle contraction is known to be promoted by lysophosphatidic acid and inhibition of ATX has been shown to decrease intraurethral pressure accompanied by urethral relaxation (see e.g. Saga et al., 2014). Therefore, targeting ATX (and its downstream LPA signaling pathways) represents a useful method for the treatment of urethral obstructive disease such as benign prostatic hyperplasia.
ATX and Pruritus
Serum ATX levels have been reported to correlate with pruritus of cholestasis (Kremer et al., 2012). Serum ATX levels have also been shown to correlate with pruritus in patients with atopic dermatitis (Nakao et al., 2014). This suggests that targeting ATX (and its downstream LPA signaling pathways) represents a useful method for the treatment of pruritus.
ATX and Hepatitis C and B/Human Hepatocellular Carcinoma
Serum ATX activity and plasma LPA levels are increased in chronic hepatitis C (HCV) in association with liver fibrosis (Watanabe et al, 2007). ATX and genes related to ATX signalling pathway were up regulated in human hepatocellular carcinoma (HCC) patients co-infected with HCV (Wu et al, 2010). It has recently been reported that ATX expression in tumour cells is specifically associated with HCV and that ATX plays a key role in HCV replication. (Reynolds et al, 2014). Recent studies have also reported the ATX-LPA signalling axis to play an essential role in the lifecycle of both chronic hepatitis B (HBV) and chronic hepatitis C (HCV) (WO2015193669). Thus, ATX-LPA is also a potential therapeutic target for the treatment of hepatitis B and hepatitis C.
ATX Inhibitors
Potent and selective ATX inhibitors are now needed as a starting point for the development of targeted anti-ATX therapy. Direct targeting of LPA receptors seems to be a less attractive strategy, since there are at least six distinct LPA receptors that show overlapping activities (see Choi et al. (2010). Since it was reported that ATX is subject to product inhibition by LPA and sphingosine-1-phosphate (S1P) (see, e.g., van Meeteren et al., 2005), various synthetic phospho- and phosphonate lipids have been explored as ATX inhibitors (see, e.g., Gajewiak et al., 2008; Cui et al, 2007; Jiang et al., 2007; Ferry et al., 2008; Zhang et al., 2009; Cui et al., 2008). However, these inhibitors have the inherent danger of inadvertently activating downstream LPA/S1P receptors, thereby inducing the opposite of the intended effect. Furthermore, lipids offer relatively few avenues for chemical diversification and usually have poor pharmacokinetic properties.
Non-lipid inhibitors of ATX have recently been identified and some of which are described in the following patents: WO2009046841; WO2009046804; WO2009046842; WO 2010115491; WO2010060532; WO2010063352; WO2010112116; WO2010112124; US2010/0016258; WO201040080; WO2011006569; WO2011044978; WO2011116867; WO2011053597; WO2011002918; WO2012166415; WO2012005227; WO2012127885; U.S. Pat. No. 8,268,891; WO2012100018; WO2013061297; WO2013054185; WO2014018881; WO2014018887; WO2014081756; WO2014152725; WO2014110000; WO2014168824; WO2014018891; WO2014025708; WO2014025709; WO2014081752; WO2014139882; WO2014143583; WO2014097151; WO 2014048865; WO2014139978; WO 2014133112.